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BASIC TERMS NEEDED TO UNDERSTAND OSMOLALITY?

To understand what osmolality is, it is first necessary to understand a few other terms. If you already know the following terms, feel free to skip to the next section.

SOLVENT: A usually liquid substance (such as water) that is capable of dissolving or dispersing one or more substances.

SOLUTE: A substance (such as sugar) that dissolves in a solvent. So if you poured sugar into a cup of water, the sugar would be the solute and the water would be the solvent.

SOLUTION: The combination of a solute with a solvent, when the solute has been evenly dissolved in the solvent. An example would be when sugar (the solute) is evenly dissolved in a glass of water (the solvent).

CONCENTRATION: A measurement of the amount of a solute compared to the amount of solvent in a solution. As an example, if a high amount of sugar (the solute) is dissolved in a small amount of water (the solvent), the concentration would be high. The concentration would be low if a small amount of sugar was dissolved in a large amount of water.

SEMIPERMEABLE MEMBRANE: A thin layer of tissue that allows some substances in, but not all. For example, the semipermeable membrane may allow a smaller substance in but not a larger one. Or it may only allow the solvent (such as water) to pass through, but not allow any solutes to pass through. Semipermeable membranes are widespread in the body and surround all cells.

WHAT IS IT?

Osmolality is the concentration of a solution. In other words, it is a measurement of the amount of a solute compared to the amount of solvent in a solution. See above for a description of some of these terms.

HOW IS OSMOLALITY EXPRESSED IN TERMS OF MEASUREMENT?

Osmolality is expressed in units of measurement known as osmoles or milliosmoles (one thousandth of an osmole), per 1000 grams (a very small unit of weight) of solvent. An osmole is the weight of the molecules (the smallest naturally occurring particles of a substance) that make up a solute, divided by the number of particles or ions (electrically charged particles) that they separate in a solution. The molecules that make up the solute are usually measured in grams.

WHAT IS THE NORMAL RANGE OF OSMOLALITY IN BLOODWORK TESTS?

Osmolality can be measured in the blood. The normal range for adults is 285 to 295 milliosmoles (abbreviated as mOsm) divided per kilograms (one thousandth of a gram) of water. However, some hospitals use a range of 280 to 300.

WHAT DOES IT MEAN IF THE OSMOLALITY IS TOO HIGH?

Osmolality can be too high for several reasons. One reason is a loss of too much water or other types of fluids from the body. Osmolality would be too high because there would be less liquid in comparison to the amount of solute (something dissolved in the liquid). Another cause would be the presence of too much salt or sugar in the blood. Uremia (a condition in which substances build up in the blood that are normally passed out of the body when one pees) can also cause a high osmolality level. The presence of poisonous levels of certain substances in the blood, such as alcohol, can cause a high osmolality level. The presence of the drug, Mannitol, which is a type of sugar that the body has difficulty breaking down, can lead to a high osmolality level because it can build up in the blood. Diabetes insipidus, which is a disease that causes too much fluid to leave the body, can also lead to a high osmolality level.

WHAT DOES IT MEAN IF THE OSMOLALITY IS TOO LOW?

Too much water or fluid in the body can cause a low osmolality level. This is because there is too much liquid in comparison to the amount of substances (solutes) that are dissolved in it. To low of an amount of salt in the blood can cause a low osmolality level. Also, a condition known as SIADH (Syndrome of Inappropriate Diuretic Hormone) can cause a low osmolality level. SIADH is an abnormal condition in which the body releases too much of a substance called antidiuretic hormone. Antidiuretic hormone works to keep water in the body. Thus, if there is too much antidiuretic hormone released, there will be too much fluid in the body.

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